US talk show host David Letterman has signed a four-year extension to his Late Show contract with CBS.

The deal will keep Letterman, 59, on air after his rival, Jay Leno, leaves NBC's Tonight Show in 2009.

Letterman has been with the CBS network since 1993 and he and Leno have been in competition for late-night US audiences ever since.

Conan O'Brien has already been named as Leno's successor but no replacement has been announced for Letterman.

During the last year, Leno's show has averaged 5.71m viewers a night compared with Letterman's 4.16m, according to Nielsen Media Research.

The news of Letterman's extended deal until the 2009 - 2010 season was issued by two TV executives who wanted to remain anonymous until the details of the deal were finalised, the Associated Press reported.

Letterman has had some health problems in recent years - he had heart surgery in 2000 and he contracted shingles in 2003 which kept him off air for a brief period.

Spokesmen for CBS Entertainment and Letterman would not comment on the negotiations.